Optical pickup devices are used for optical digital audio disc players or optical video disc players to reproduce information recorded on a disc. In the optical pickup of a digital audio disc player, a semiconductor laser is used as a light source to minimize the size and weight of the optical pickup.
Track following control methods are currently employed to eliminate errors due to warped discs, disc surface irregularities and eccentric tracks. In a first method, the optical pickup objective lens system 1 is moved in the direction of focus indicated by arrow F in FIG. 1 and in directions transverse to the tracks as indicated by arrow T. In a second method the objective lens system 1 is moved in focus direction F and the whole unit of pickup 2 including the laser, light sensor and the lens, is moved in the transverse direction T as shown in FIG. 2. In a third method, the whole unit 2 is moved in both the F and T directions.
Of these, the first and second methods have been employed extensively. In the conventional methods, including the third method, beams incident on the objective lens 1 must be parallel, to enable the objective lens 1 to be controlled in all directions without decreasing its optical performance; commercially available objective lenses are designed to fit parallel ray arrangements.
The following is a description of conventional optical pickups.
FIG. 4 is a side view of a conventional optical pickup including semiconductor laser 3, collimating lens 4, semitransparent mirror 5, objective lens 6, objective-lens drive unit 7, an error detecting optical system 8 for detecting focus error and tracking error, a light-receiving element 9, and a disc 10.
The operation of this optical pickup is as follows.
Light emitted, from the semiconductor laser 3 is collimated by the collimating lens 4 before reaching the objective 6 through the semitransparent mirror 5 and then focused on the disc 10. Light reflected onto disc 10 is collimated by lens 6 and is divided by the semitransparent mirror 5 into two beams, one of which is focused on the light-receiving element 9 by the error detecting optical system 8.
The objective lens 6 is controlled by the objective drive unit 7 in the focussing and tracking directions so that the track following control can be performed to minimize errors due to warped discs, eccentric tracks dr the like.
The collimating lens 4, shown in FIG. 5, is positioned between semitransparent mirror 5 and objective 6, resulting in a compact structure as compared with that shown in FIG. 4.
Since the distance between semiconductor 3 and collimating lens 4 and the distance between objective 6 and disc 10 are respectively determined by such factors as the focal length of the respective lenses, and the working distances to reduce the size of the optical system, it is necessary for the distance between collimating lens 4 and objective lens 6 to be as short as possible.
One possible solution is to reduce the height of the objective drive unit 7. However, because of the design limitations imposed on the maximum working distance of objective 6, as seen in FIG. 6, (which limits the allowable distance between it and disc 10), and because the distance between disc 10 and cover 13 and the distance between cover 13 and a lens holder 11 must be greater than the movable ranges of disc 10 and lens 6, lens 6 must extend upward from lens holder 11. This requires a balance weight 12 to be attached to the lower part of the lens holder to counter the weight of the lens 6 to ensure stability of the moving part of the unit, making it difficult to reduce the overall height of the unit. Further design considerations given to the stability and driving power of the unit necessarily result in a bulky structure, that is 15 mm to 20 mm high.
Illustrated in FIG. 6 are coils 14a and 14b magnets 15a and 15b and yokes 16a and 16b. These parts form a magnetic circuit for driving the objective 6. In the same drawings, members 17a, 17b, 17c and 17d support the moving part of the unit; base 18 is provided for the objective drive unit.
In conventional optical pickups described above, a costly collimating lens system is additionally required for collimating the diverging light rays emitted from semiconductor laser 3 in a manner shown in FIG. 7. This increases the total cost of the drive unit and the number of parts comprising the unit, resulting in a larger aberration of the whole system.
Furthermore, the collimating lens system 4 has a relatively long focal length, generally in the range from 14 mm to 17 mm. If this focal length is taken into account, the distance between laser 3 and disc 10 would inevitably become large and size reduction, particularly height reduction, would be impossible to achieve.
One prior attempt to achieve height reduction is shown in FIG. 8 in which a total-reflection prism 19 is provided in the path of a beam from laser 3 to objective 6. However, this increases the total aberration of the optical system and requires prism 19 to be adjusted in position to correct errors which might occur during assemblage and further requires the laser 3 to be adjusted in position for alignment purposes. Thus, the use of a prism is undesirable from the manufacturing standpoint.
A further disadvantage of conventional pickups is that objective lenses are designed for use with collimated light rays. Thereby, objective lens 1, FIG. 9, can only move in a range corresponding to the difference in diameter between it and collimating lens system 4. An increase in the diameter of collimating lens system 4, in an attempt to extend the movable range of lens 1, would result in a greater focal length and a larger numerical aperture.